Connect copper to steel pipes

Thank you all for responding to my post. My house is cold so I need to move forward soon!

My plan is to use a Trinity 150 combi with an indirect tank for DHW. Then, for heat, six high temp zones, one for the basement (mostly above ground), one to include both the living room and dining room which contains the only thermostat, one for the kitchen, and one for each of three bedrooms. Each radiator, except for LR and DR will have a TRV. The system will have a Pressure Bypass Differential Valve. The six zone manifold will be run off one circulator pump.

I may instead decide to put the basement on its own circulator pump with it's own thermostat.

Since I'm installing TRV's anyway, I want to get rid of the iron pipe which will also solve some architectural issuse. I'll reach each radiator with 3/4" Pex-AL-Pex unless I can be convinced to run 1/2".

I'm unsure if I should put balancing valves on the manifold or not. The zones will be unequal.

I'll be replacing a 100K BTU standard boiler. The water temp must have been (boiler is totally dead now) high, probably 180 or so. someone suggested 130 degree water would do fine. Although this would allow for higher modulation I'm not sure if I can get adequate BTUs at that temp.

Comments

I need to connect to old radiator pipes for my boiler replacement project. The pipe system was steam then converted to hot water. Now I'm going to zone the system with circulator pumps. I'll need to connect to the existing pipes in some way. Do I have to use an existing, threaded joint the connect with a brass fitting or can I somehow cut the pipe and connect with a compression fitting?

Some of the pipes are 1.75", 1.25" and .75". Is there a good online source for brass fittings in these sizes?

Save yourself the time and just pipe in a PBDV and some TRV's. Make every room its own zone, and be done with it. With a MOD/CON boiler, the pipe and water content become your friend. Look into it. The upfront cost is without a doubt higher but the payback will come way quicker. JMHO. Chris

I still have to change the connection to the radiator from the iron pipe to something to which I can connect the PEX so I don't how I save on the issues you mentioned (Save fittings, time, routing, soldering, size and dielectric issues in one job. What do you think?)

I still like the idea of zoning with circulator pumps rather than zone valves. One zone for each of three floors plus one for DHW. My house is small so I think all the thermostats (one for each room) would be excessive.

Do you think It's wrong to use 3/4" PEX?

My new boiler will modulate between 31,000BTU and 138,000BTU so I don't think oversizing is an issue.

are really like compression fittings with male thread ends. Think of it as a bushing installation. Nothing to it. Just connect at the nearest female joint and you are done. I prefer the PEX-AL-PEX for durability.

As far as the "jumping off point" from your new system, consider a manifold (Alberta Custom Tee among others) whereby you can have a valve for each coupling and off you go.

TRVs aren't "thermostats" in the sense you're probably thinking. Instead of being "on" or "off" they automatically vary flow to produce the desired room temp setting. This is called proportional control. TRVs are self-contained and require no wiring or electricity.

In most weather conditions, all of the TRVs will be "somewhat on". This makes for an ideal load when using a modulating boiler as it will operate to satisfy the entire system simultaneously instead of "zones" that are either calling or not calling for heat.

Oversizing is ALWAYS an issue. You really, really, really need a good heat loss calculation and size the boiler accordingly. While that 138,000 btu boiler may modulate down to 31,000 btu, your house does not present a constant load. As the weather gets warmer, the heat loss diminishes. If the boiler is oversized, the house will be loosing less than the minimum output more and more of the time--thus you get less and less true modulation.

The water being bypassed because of room satisfaction needs a "relief"(if you will). If it can't flow through the radiators that are satisfied, it will have to go somewhere. This is where the Pressure Bypass Differential Valve comes into play. When properly set, it will just send the unused heating water back to the boiler to reduce the load. Most MOD/CON boilers work their magic based on temperature going out VS. the temperatures coming back.(outdoor reset merely tells the boiler which temperature will work the best for the programmed load and design temps.)

This is the preffered method in my opinion. The rest of the world does it with lots of the design criteria being to get the appropriate amount of heat for the space...with 130° water temperatures. With big radiators, this is very attainable as most were designed for design day calculations, with the WINDOWS OPEN!!! Food for thought? Chris

That size will handle 1.0 GPM comfortably, which is 10,000 BTU's per hour at a 20 degree drop. If a given radiator is even 5,000 that can be a lot. I have half inch connections to most of my radiators and they run fine. I also run a 30 degree delta-T on most so my flow rates are lower, granted. But if I want to increase flow a bit, not an issue with delivery, noise or anything else.

My conservative side said "3/4 inch minimum" but that is really not necessary for a single runout to a radiator. Either way you will be fine.

Just keep in mind that your TRV's want to be sized for expected flow rate so they maintain proper authority. Final connection size issue is all that is.